Re: [PATCH 1/2] mm: fadvise: fix POSIX_FADV_DONTNEED

[Dave Hansen <dave@linux.vnet.ibm.com>]

On 02/24/2013 02:38 PM, Phillip Susi wrote:
> On 02/24/2013 04:25 PM, Dave Hansen wrote:
>> Essentially, they don't want any I/O initiated except that which
>> is initiated by the app.  If you let the system get in to reclaim,
>> it'll start doing dirty writeout for pages other than those the app
>> is interested in.
> 
> Are you talking about IO initiated by the app, or other tasks in the
> system?  If the former then it won't be affected by this change.

Once we go in to reclaim, we'll start writeback on dirty pages.  The
VM's writeback patterns are not as efficient as if the app itself was
initiating them from sync_file_range(), and we see massive throughput
loss on the disk.

>From the looks of your patch, deactivating all the pages (if clean of
course) will get them on the LRU, and should keep any direct-reclaiming
tasks from actually going and initiating any additional I/O.

It looks like a promising approach, at least theoretically.  I'll
definitely test and see if it works in practice.

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